Bulk Optical Properties

This document describes the optical properties that can be associated with bulk (volume) materials in a DIRSIG Material Database file.

Absorption Coefficient Properties

Absorption coefficient properties are supplied via the ABSORPTION_PROP in the BULK_PROPERTIES of the material file.

Classic Absorption

Background

The ClassicAbsorption model was implemented in DIRSIG4 as a method to provide backwards compatibility (in terms of both input file format and mathematical behavior) to the spectral absorption support in DIRSIG2 and DIRSIG3. Historically this property was used when modeling plume gases, where the absorption was proportional to the gas concentration.

Setup and Options

The ClassicAbsorption model is configured with an external ASCII/Text containing spectral absorption coefficients with units of 1/ppm-m.

ABSORPTION_PROP_NAME = ClassicAbsorption
ABSORPTION_PROP {
    FILENAME = plume.abs
}

Input File

The format of the spectral absorption file provided via the FILENAME variable is described in detail in a separate document.

Examples

The RegularGrid1 demo contains an example of using this property.
The ExtinctionProp1 demo contains multiple examples of using this property.

References

None.

Scattering Coefficient Properties

Scattering coefficient properties are supplied via the SCATTERING_PROP in the BULK_PROPERTIES of the material file.

Simple Scattering

Background

This optical property was introduced in DIRSIG Release 4.6 as an easy way to import spectral scattering coefficient data.

Setup and Options

The SimpleScattering model is configured with an external, ASCII/Text file containing the spectral scattering coefficient with units of 1/m.

SCATTERING_PROP_NAME = SimpleScattering
SCATTERING_PROP {
    TXT_FILENAME = cumulus.txt
}

Input File

The input file is currently assumed to be a two-column, space delimited (space or tab), ASCII/Text file containing wavelengths in microns and the corresponding scattering coefficient. The spectral data is linearly interpolated to compute wavelengths between those provided.

 0.20 0.10
...
[lines deleted for documentation purposes]
...
20.00 0.10

Examples

None.

References

None.

Phase Function Properties

Phase function properties are supplied via the PHASE_FUNCTION_PROP in the BULK_PROPERTIES of the material file.

Henyey Greenstein

Background

This optical property was introduced in DIRSIG Release 4.6 as an easy way to use the classic Henyey and Greenstein (HG) scattering phase function.

\begin{equation*} p(\theta) = \frac{1}{4\pi} \frac{1 - g^2}{[1 + g^2 + 2g\cos(\theta)]^{3/2}} \end{equation*}

This property allows you to supply the HG asymmetry parameter (g) as a function of wavelength.

Setup and Options

The HenyeyGreenstein model is configured with an external, ASCII/Text file containing the spectral asymmetry parameter.

PHASE_FUNCTION_PROP_NAME = HenyeyGreenstein
PHASE_FUNCTION_PROP {
    TXT_FILENAME = cloud_asym.txt
}

Input File

The input file is currently assumed to be a two-column, space delimited (space or tab), ASCII/Text file containing wavelengths in microns and the corresponding asymmetry parameter. The spectral data is linearly interpolated to compute wavelengths between those provided.

 0.20 0.80
...
[lines deleted for documentation purposes]
...
20.00 0.50

Examples

None.

References

Henyey, L.C. and J.L. Greenstein., "Diﬀuse radiation in the galaxy", Journal of Astrophysics, Vol 93, pp. 70-83, 1941